Lithium disilicate glass ceramics

ABSTRACT

This invention is directed to lithium disilicate (Li 2 Si 2 O 5 ) based glass-ceramics comprising silica, lithium oxide, alumina, potassium oxide and phosphorus pentoxide. The glass-ceramics are useful in the fabrication of single and multi-unit dental restorations (e.g. anterior bridges) made by heat pressing into refractory investment molds produced using lost wax techniques. The glass-ceramics have good pressability, i.e., the ability to be formed into dental articles by heat-pressing using commercially available equipment. In accordance with one embodiment directed to the process of making the glass-ceramics, the compositions herein are melted at about 1200° to about 1600° C., thereafter quenched (e.g., water quenched or roller quenched) or cast into steel molds, or alternately, cooled to the crystallization temperature. The resulting glass is heat-treated to form a glass-ceramic via a one or two step heat-treatment cycle preferably in the temperature range of about 400° to about 1100° C. The resulting glass ceramics are then pulverized into powder and used to form pressable pellets and/or blanks of desired shapes, sizes and structures which are later pressed into dental restorations. Alternatively, instead of forming into pressable pellets or blanks, the pulverized powder is used to form a dental restoration using the refractory die technique or platinum foil technique.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. application Ser. No.09/458,919, filed Dec. 10, 1999, now U.S. Pat. No. 6,455,451, whichclaims priority to U.S. Provisional Application No. 60/153,916, filedSep. 14, 1999, U.S. Provisional Application No. 60/122,558, filed Mar.2, 1999, and U.S. Provisional Application No. 60/111,872, filed Dec. 11,1998, all which are incorporated herein by reference.

FIELD OF INVENTION

This invention relates generally to glass-ceramics comprising lithiumdisilicate and more specifically to glass-ceramics for use in themanufacture of dental restorations and methods of manufacture thereof.

BACKGROUND OF THE INVENTION

The use of lithium disilicate glass ceramics for use in dentalrestorations has been suggested in the prior art. U.S. Pat. No.4,189,325 to Barret et al. is directed to a glass-ceramic comprisinglithium disilicate for use in dental restorations. The glass ceramicrequires the presence of Nb₂O₅ and Pt as nucleation agents. Barrett etal. introduced dental restorations made from castable lithium disilicateglass-ceramics in the Li₂O-CaO-Al₂O₃-SiO₂ system nucleated by Pt andNb₂O₅. According to Barret et al., dental restorations are made bycasting a melt into an investment mold, and devitrifying thereafter.

U.S. Pat. No. 4,515,634 to Wu et al. is directed to a castableglass-ceramic composition wherein the glass is melted and cast into ashape and is crystallized after it has been shaped. Therefore, thecrystallization process is performed by the technician making therestoration, not the manufacturer of the dental material. Wu set al.suggests one way to improve properties of castable lithium disilicatedental restorations within the same Li₂O-CaO-Al₂O₃-SiO₂ system asdescribed by Barrett et al. is by utilization of P₂O₅ as a nucleatingagent. Both Barrett et al. and Wu et al. describe castable compositionshaving CaO as an essential ingredient believed to improve chemicaldurability of the resulting glass-ceramics. Chemical durability is oneof the major issues that the Wu and Barrett inventions fail to address.For example, total alkali leaching rates for materials presented in Wu'sexamples were four to five times higher than those for commercial dentalporcelain.

Castable dental ceramics as described in Barret et al. and Wu et al.employ melting glass ingots supplied by a manufacturer and castingdental articles into a refractory investment mold. Following the castingprocess, the cast articles are devitrified (crystallized) by therequired heat-treatment steps. This process is very similar to castingmetals whereby a heat-treatment step follows the casting process toincrease hardness and strength.

U.S. Pat. Nos. 5,507,981 and 5,702,514 to Petticrew teach lithiumdisilicate compositions for use in dental restorations, but the methoddescribed therein implies forming glass into the shape of a dentalrestoration at temperatures much higher than the melting temperature oflithium disilicate and heat-treating the dental restoration afterforming to convert the glass into a glass-ceramic.

German Patent Application No. DE19647739 to Schweiger et al. is directedto lithium disilicate compositions for use in dental restorations. Theglass-ceramic bodies or blanks used to press dental restorations aredefined as “sinterable glass ceramics” which are produced from thestarting amorphous glass powder by simultaneous sintering and powdercrystallizing, which process is also known as surface crystallization.The glass must be in powder form to be crystallized. Additionally, thelithium disilicate compositions therein require the presence of La₂O₃,MgO and ZnO.

Many of the lithium disilicate compositions in the prior art requirecasting of the glass into the desired shape and crystallizingthereafter. The glass must be formed into the finally desired shape andthereafter heat treated to crystallize into a lithium disilicate phase.This may result in structural and other problems, since themicrostructure is not formed by the dental materials manufacturer, butby the technician fabricating the dental restoration. Overprocessing bya technician may change the microstructure of the material to somethingnot preferred or desired by the dental materials manufacturer. Moreover,some of the prior art compositions require the forming of the glassceramics by surface crystallization, limiting the forming andcompositional possibilities of the material.

It is desirable to provide a lithium disilicate glass-ceramic which ispressable or formable after the lithium disilicate is formed. It isbeneficial to provide a lithium disilicate glass ceramic for use in thefabrication of dental restorations wherein crystallization is carriedout by the dental materials manufacturer in the most controlled manner.It is beneficial to provide translucent lithium disilicate glassceramics having high strength and good presssability.

SUMMARY OF THE INVENTION

This invention is directed to lithium disilicate (Li₂Si₂O₅) basedglass-ceramics comprising silica, lithium oxide, alumina, potassiumoxide and phosphorus pentoxide in addition to other components listedbelow. The glass-ceramics are useful in the fabrication of single andmulti-unit dental restorations including but not limited to orthodonticappliances, bridges, space maintainers, tooth replacement appliances,splints, crowns, partial crowns, dentures, posts, teeth, jackets,inlays, onlays, facing, veneers, facets, implants, abutments, cylinders,and connectors made by a variety of techniques including heat pressinginto refractory investment molds produced using lost wax techniques orbuilding and sintering powder onto refractory dies such as in therefractory die/foil technique. The glass-ceramics have goodpressability, i.e., the ability to be formed into dental articles byheat pressing, also known as hot pressing, or injection molding, usingcommercially available equipment and good formability, i.e., the abilityto be applied in powder form to a dental model, i.e., on a refractorydie, and heated to form a dental restoration.

In accordance with one embodiment directed to the process of making theglass-ceramics, the compositions herein are melted at about 1200° toabout 1600° C. and preferably in the range of about 1300° to about 1400°C. for a period of time, preferably for about 4 hours and thereafterquenched (e.g., water quenched or roller quenched) or cast into steelmolds, or alternately, cooled to the crystallization temperature.

The resulting glass is heat-treated to form a glass-ceramic via a one ortwo step heat-treatment cycle preferably in the temperature range ofabout 400° to about 1100° C. This crystallization heat treatment maycomprise a nucleation step and a crystal growth step. Depending on thecomposition, the first, nucleation step, may be carried out in the rangeof about 450° C. to about 700° C. and preferably in the range of about500° C. to about 650° C. for about 0.5 to about 4 hours and the second,crystal growth step, may be carried out in the range of about 800° C. toabout 1000° C. and preferably in the range of about 830° C. to about930° C. for about 0.5 to about 48 hours. The most preferable heattreatment comprises about a one hour soak at about 645° C. and asubsequent four hour soak at about 850° C.

The resulting glass ceramics are then pulverized into powder and used toform pressable pellets and/or blanks of desired shapes, sizes andstructures. Additives may be mixed with the powder prior to forming intopellets or blanks. These pellets and blanks may be used for pressingcores or other frameworks or shapes for dental restorations. The blankor pellet may be subjected to viscous deformation at a temperature inthe range of about 800° to about 1200° C., and more preferably in therange of about 850° to about 950° C., and most preferably at less thanabout 930° C., under vacuum and with the application of pressure ofbetween about 2 to about 8 bar (0.2 to 0.8 MPa) and preferably nogreater than about 6 bar (0.6 MPa) to obtain a dental restoration.Moreover, it is possible that the blanks may be machined to a dentalrestoration of desired geometry.

Alternatively, instead of forming into pressable pellets or blanks, thepulverized powder, with or without additives, is used to form a dentalrestoration using the refractory die technique or platinum foiltechnique.

BRIEF DESCRIPTION OF THE DRAWINGS

Features of the present invention are disclosed in the accompanyingdrawing, wherein:

FIG. 1 s perspective view of a plunger system in a pressing furnace foruse in the fabrication of dental restorations in accordance with oneembodiment of the invention.

DESCRIPTION OF THE INVENTION

As will be appreciated, the present invention provides glass-ceramiccompositions comprising a glassy matrix and lithium disilicate(Li₂Si₂O₅). The glass-ceramics are useful for the fabrication of dentalrestorations. The compositions of the lithium disilicate glass-ceramicscomprise inter alia, silica, lithium oxide, alumina, potassium oxide andphosphorus pentoxide in the ranges given in Table 1 below. Theglass-ceramic compositions of the invention have a combination ofproperties useful for dental restorations. The glass-ceramics have goodpressability, i.e., the ability to be formed into dental articles byheat pressing, also known as hot pressing, or injection molding, usingcommercially available equipment. The glass-ceramics also have goodformability, i.e., the ability to be applied in powder form to a dentalmodel and heated to form a dental restoration.

Pressable ceramics employ some form of hot-pressing or injection-moldingof the glass-ceramic materials, which are typically in the form ofpellets. The pellets contain one or more crystalline phases and theirmorphology as well as volume fraction are not significantly altered inthe course of pressing. One reason for this is that the pressingtemperature is typically lower than the melting temperature of thecrystalline phases. This is a major advantage because microstructure isformed in the controlled conditions by the manufacturer of theglass-ceramic materials, e.g., pellets. Following pressing, theresulting dental article does not require crystallizationheat-treatment. Similarly, the powder glass-ceramics contain one or morecrystalline phases and their morphology and volume fraction are notsignificantly altered in the course of sintering during the fabricationof a dental article.

The glass-ceramic pellets can be formed by a number of processes: (1)Glass can be cast into the shape of pellet. Pellets are taken from themold and crystallized. These pellets can not be shaded by the additionof pigments. (2) Glass can be crystallized in bulk and subsequentlymilled into powder. Pigments and other additives can be added to thepowder. The powder is formed into a pellet and partially or fullysintered. Pigments, if added, create color centers that impart a certaincolor to a translucent body of the dental article pressed from thepellet. The mechanism of crystallization in the two processes describedabove is volume crystallization. Volume crystallization as described inprocess two above may be used to form glass-ceramic powder withoutforming into a pellet.

Alternatively, surface crystallization may be utilized to crystallize aportion of the glass into one or more crystal phases. This involvesmilling glass into powder. Pigments and other additives can be added tothe powder. This glass powder (amorphous, not crystalline) is formedinto a pellet. The glass pellet is sintered and crystallized in the samefiring cycle. Not all glass-ceramic materials can be crystallized andsintered simultaneously. Only certain materials and compositions proneto surface crystallization can be processed this way. The glass-ceramicsprocessed from glass powder via simultaneous sintering andcrystallization are sometimes called “sintered” glass-ceramics. Anotherterm that can be used is “sinterable” glass-ceramics.

The compositions herein are prepared by mixing, in the desiredproportions, the oxides and/or compounds that decompose to form theoxides, followed by fusing the ingredients to obtain the compositions inTable 1. Convenient raw material include lithium carbonate, silica,alumina, carbonates of K, Na, Ca, ammonium phosphate, tricalciumaluminate, aluminum phosphate or aluminum metaphosphate and ifnecessary, Ta₂O₅, CeO₂, Tb₄O₇, titanium dioxide, and zirconium dioxide.

TABLE 1 Oxide, wt % Range 1 Range 2 Range 3 Range 4 Range 5 Range 6 SiO₂about 62 to about about 64 to about about 62 to about about 64 to aboutAbout 64 to about about 62 to about 85 70 85 70 70 76 B₂O₃ 0 to about4.9 0 -to about 2.7 0 to about 4.9 0 to about 2.7 0.5 to about 3.0 0 toabout 5 Al₂O₃ about 1.5 to about about 1.5 to about about 5.1 to aboutabout 5.2 to about about 1.5 to about about 1.5 to about 10 6.0 10 9.06.0 10 F 0 to about 1.5 0 to about 1.5 0 to about 1.5 0 to about 1.5 0to about 1.5 0 to about 1.5 ZnO 0 to about 5 0 to about 2 0 to about 5 0to about 2 — 0 to about 5 CaO 0 to about 7 0 to about 0.9 0 to about 7 0to about 0.9 0 to about 0.9 0 to about 7 MgO 0 to about 2 0 to about 2 0to about 2 0 to about 2 — 0 to about 2 BaO 0 to about 7 0 to about 7 0to about 7 0 to about 7 0 to about 7 0 to about 7 SrO 0 to about 1 0 toabout 1 0 to about 1 0 to about 1 0 to about 1 0 to about 1 Cs₂O 0 toabout 5 0 to about 5 0 to about 5 0 to about 5 0 to about 5 0 to about 5Li₂O about 8 to about about 10 to about about 8 to about about 10 toabout about 10 to about about 8 to about 19 15 19 15 15 19 K₂O about 2.5to about about 2.5 to about 0 to about 7 0 to about 5 about 2.2 to about0 to about 7 7 5 5 Na₂O 0 to about 5 0 to about 3 0 to about 5 0 toabout 3 about 0.5 to about 0 to about 5 3 TiO₂ 0 to about 2 0 to about 20 to about 2 0 to about 2 0 to about 2 0 to about 2 ZrO₂ 0 to about 3 0to about 3 0 to about 3 about 0 to about 3 0 to about 3 0 to about 3P₂O₅ about 0.5 to about about 2 to about 7 about 0.5 to about about 2 toabout 7 about 2 to about 7 about 0.3 to about 12 12 7.0 SnO₂ 0 to about1 0 to about 1 0 to about 1 0 to about 1 0 to about 1 0 to about 1 Sb₂O₃0 to about 1 0 to about 1 0 to about 1 0 to about 1 0 to about 1 0 toabout 1 Y₂O₃ 0 to about 3 0 to about 3 0 to about 3 0 to about 3 0 toabout 3 0 to about 3 CeO₂ 0 to about 1 0 to about 1 0 to about 1 0 toabout 1 0 to about 1 0 to about 1 Eu₂O₃ 0 to about 1 0 to about 1 0 toabout 1 0 to about 1 0 to about 1 0 to about 1 Tb₄O₇ 0 to about 1 0 toabout 1 0 to about 1 0 to about 1 0 to about 1 0 to about 1 Nb₂O₅ 0 toabout 2 0 to about 2 0 to about 2 0 to about 2 0 to about 2 0 to about 2Ta₂O₅ 0 to about 2 0 to about 2 0 to about 2 0 to about 2 0 to about 2about 0.5 to about 8.0

For each of the compositions in Table 1, the molar ratio of(Na₂O+K₂O+CaO+SrO+BaO)/(Al₂O₃+ZnO)≧1.3. The compositions in Table 1 aremelted at about 1200° to about 1600° C. and preferably in the range ofabout 1300° to about 1400° C. for a period of time, preferably for about4 hours and thereafter quenched (e.g., water quenched or rollerquenched), cast into steel molds, or alternately, cooled to thecrystallization temperature. If the melt is cooled to thecrystallization temperature, it may remain in the same furnace.

The resulting glass is heat-treated to form glass-ceramics using a oneor a two step heat-treatment cycle preferably in the temperature rangeof about 400° to about 1100° C. This crystallization heat-treatment maycomprise a nucleation step and a crystal growth step. Depending on thecomposition, the first, nucleation step, may be carried out in the rangeof about 450° C. to about 700° C. and preferably in the range of about500° C. to about 650° C. for about 0.5 to about 4 hours and the second,crystal growth step, may be carried out in the range of about 800° C. toabout 1000° C. and preferably in the range of about 830° C. to about930° C. for about 0.5 to about 48 hours. The most preferable heattreatment comprises about a one hour soak at about 645° C. and asubsequent four hour soak at about 850° C.

The glass ceramics comprise lithium disilicate. The resulting glassceramics are then pulverized into powder sieved to −200 mesh to providepowder with average particle sizes of about 30 to about 40 microns.Pigments, fluorescing agents, opacifying agents, and the like may beadded to the powder in a wide range in an amount between about 0 andabout 6 wt % and preferably in the amount of between about 0 and about 5wt % and most preferably in the amount of about 0% to about 3 wt %.Moreover, reinforcing agents may be added to the powder in an amount offrom about 0 to about 30 vol % and more preferably in an amount of fromabout 0 to about 20 vol %. The reinforcing agents may include fibers,whiskers, and particulate fillers and may be fabricated of any knownmaterial, preferably a glass or ceramic material.

The powders may be used in powder form to produce a dental material ormay be used to form and fuse pressable pellets and/or blanks of desiredshapes, sizes and structures. These pellets and blanks may be used forpressing cores or other frameworks or shapes for dental restorations.The blank or pellet may be subjected to viscous deformation at atemperature in the range of about 800° to about 1200° C., and morepreferably in the range of about 850° to about 950° C., and mostpreferably at less than about 930° C., under vacuum and with theapplication of pressure of between about 2 to about 8 bar (0.2-0.8 MPa)and preferably no greater than about 6 bar (0.6 MPa) to obtain a dentalrestoration. Moreover, it is possible that the blanks may be machined toa dental restoration of desired geometry using commercially availablemilling equipment such as the Maho HGF 500 5 Axis CNC Milling Machineavailable from Fraunhofer Institut Produktionstechnologie, Germany. Whenpowder is used to fabricate dental restorations, the powder is appliedto the mold using refractory die techniques or the platinum foiltechnique. The powder is formed onto a die to form a dental restorationand is sintered.

In an alternative method herein, the compositions in Table 1 are meltedat about 1200° to about 1600° C. and preferably in the range of about1300° to about 1400° C. for a period of time, preferably for about 4hours and thereafter water quenched or cast into steel molds.

The quenched glass is comminuted to a powder. Pigments, fluorescingagents, opacifying agents, and the like may be added to the powder in awide range in an amount between about 0 and about 6 wt % and preferablyin the amount of between about 0 and about 5 wt % and most preferably inthe amount of about 0% to about 3 wt %. Moreover, reinforcing agents maybe added to the powder in an amount of from about 0 to about 30 vol. %and more preferably in an amount of from about 0 to about 20 vol. %. Thereinforcing agents may include fibers, whiskers, and particulate fillersand may be fabricated of any known material, preferably a ceramicmaterial.

The powder is compacted into a pellet or starting blank. The blank isthereafter simultaneously sintered and crystallized. Heat treatment maybe one or more cycles in the temperature range of about 400° to about1100° C. The crystallization may comprise a nucleation step and acrystal growth step. Depending on the composition, the first, nucleationstep, may be carried out in the range of about 450° C. to about 700° C.and preferably in the range of about 500° C. to about 650° C. for about0.5 to about 4 hours and the second, crystal growth step, may be carriedout in the range of about 800° C. to about 1000° C. and preferably inthe range of about 830° C. to about 930° C. for about 0.5 to about 48hours. The most preferable heat treatment comprises about a one hoursoak at about 645° C. and a subsequent four hour soak at about 850° C.

The blank or pellet may be subjected to viscous deformation at atemperature in the range of about 800° to about 1200° C., and morepreferably in the range of about 850° to about 950° C., and mostpreferably at less than about 930° C., under vacuum and with theapplication of pressure of about between about 2 to about 8 bar (0.2-0.8MPa) and preferably no greater than 6 bar (0.6 MPa) to obtain a dentalrestoration. Moreover, it is possible that the blanks may be machined toa dental restoration of desired geometry.

To achieve the required combination of properties, namely sufficientstrength, formability below 950° C., by heat-pressing using commerciallyavailable dental presses such as the Autopress®) Plus available fromJeneric/Pentron, Wallingford, Conn., translucency and chemicaldurability, the optimal chemical combinations and crystallizationtreatment of the present invention are necessary. The best propertiesare obtained when the lithium metasilicate (Li₂SiO₃) and silica phasesare nearly absent, the volume fraction of Li₃PO₄ is less than about 5%and the volume fraction of lithium disilicate (Li₂Si₂O₅) is betweenabout 35% and about 60%. High aspect ratio morphology of the lithiumdisilicate phase is important and is believed to enhance mechanicalproperties, i.e., strength and fracture toughness of the glass-ceramic.

Li₂O and SiO₂ are instrumental in crystallizing the required amount ofthe lithium dislicate phase in the compositions of the presentinvention. Additionally, BaO and Cs₂O stabilize the residual glass andboost the refractive index of the residual glass to match that oflithium disilicate. Al₂O₃ and to a lesser extent, B₂O₃, if less than 3%,yield chemically durable glass-ceramics that exhibit a sufficiently lowsolubility. Alkali (Na, K, Cs) and alkaline earth metal (Ca, Ba) oxidesare required to lower processing temperatures of the glass-ceramic.However some of them affect chemical durability more than others. Withrespect to the following group of alkali metals and alkaline earthmetals of potassium, calcium, sodium, and barium; potassium isassociated with the smallest decrease in chemical durability of lithiumcontaining glasses, with calcium being next, and sodium and bariumaffecting chemical durability the most. However, the best combination ofproperties is achieved when those oxides are used in combination toachieve the so-called “mixed alkali effect.” F lowers the viscosity ofthe glass-matrix and enhances formability at temperatures below about950° C. Y₂O₃ in combination with Ce₂O₃, Eu₂O₃ and Tb₄O₇ modify therefractive index as well as impart fluorescence. Nb₂O₅ and Ta₂O₅ modifythe refractive index as well as aid nucleation and chemical durabilityof the resulting glass ceramics.

The following Table 2 illustrates examples of the compositions of theinvention.

TABLE 2 Oxide, wt % Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex. 7 Ex. 8 Ex.9 Ex. 10 SiO₂ 68.8 70.0 70.1 68.1 69.5 68.8 65 67.2 68.9 72 B₂O₃ 1.2 1.0— — 1.3 1.3 1.0 1.2 0.9 2.0 Al₂O₃ 4.8 4.9 5.2 4.7 4.8 4.8 5 4.7 47 4.5 F— — — — — — — — — — ZnO — — — — — — — — — — CaO 0.9 0.5 2.2 2.0 2.0 1.01 1 0.5 0 MgO — — — — — BaO 2.8 1.4 — — — 2.8 2.7 2.7 1.4 2.0 SrO — — —— — — — — — — Cs₂O — — — — — — — — — — Li₂O 14.4 14.7 14.7 14.3 14.614.4 15 14.1 14.3 13.0 K₂O 2.5 2.5 4.6 4.4 4.5 2.2 2.2 2.2 2.0 2.0 Na₂O1.4 1.4 — — — 1.5 1.4 1.4 1.3 — TiO₂ — — — — — — — — — — ZrO₂ — — — — —— — — — — P₂O₅ 3.3 3.6 3.4 3.3 3.4 3.3 3.5 3.2 3.5 3.0 SnO₂ — — — — — —— — — — Sb₂O₃ — — — — — — — — — — Y₂O₃ — — — — — — — — — 0.5 CeO₂ — — —0.4 — — 0.4 0.4 0.3 0.6 Eu₂O₃ — — — — — — — — — 0.6 Tb₄O₇ — — — 0.8 — —0.9 — 0.3 — Nb₂O₅ — — — — — — — — — — Ta₂O₅ — — — 2.0 — — 1.9 2.0 1.8 —Molar ratio 1.772 1.398 1.727 1.787 1.772 1.777 1.66 1.765 1.307 0.777of (Na₂O + K₂O + CaO + SrO + BaO): (ZnO + Al₂O₃) Three- 420 ± 440 ± 400± Point 60 60 50 Flexural Strength per ISO 6872, MPa As-pressed 42 25 34Opacity (relative opacity units) CTE 10.4 10.1- 10.5 10.0 9.9 (25° C.-10.6 500° C.), 10⁻⁶° C.⁻¹

The following examples illustrate the invention.

EXAMPLE 1

Glass-ceramic compositions of the present invention were utilized tomake glass-ceramic pellets. These pellets were used to make rectangularbars and rods for measuring the flexural strength by heat-pressing thesepellets into the cavity of the refractory investment mold. The processwas the same as that used to make dental restorations. The three-pointflexure strength was greater or equal to 300 MPa. This strength issufficient for multi-unit restorations such as anterior bridges. Thecompositions have a significant amount of glass phase, i.e.,approximately 50% glass phase which enhances the pressability thereof.

Moreover, the closeness of refractive indices of the matrix glass (˜1.5)and that of the crystallized phase—lithium disilicate—(˜1.55) allows forthe possibility of translucent glass-ceramics. Specifically in thepresent invention, the refractive index of the glass matrix is increasedto match that of the lithium disilicate phase by adding small amounts ofheavy ions such as, but not limited to, Sr, Y, Nb, Cs, Ba, Ta, Ce, Euand Tb.

The following examples in Table 3 illustrate the effect of differentadditions on translucency, strength and reactivity with investment ofthe resulting glass ceramics. The composition of Example 6 (Table 2) wasselected as a control composition for the purposes of this study. Thiscomposition was modified by adding 0.13 mole% of CeO₂, or 0.06 mole% ofTb₄O₇, or 0.26 mole% of Ta₂O₅, or La₂O₃, or Y₂O₃; and combinations ofthe latter with CeO₂. Opacity was measured on the pressed disk using anoptical densitometer. Reactivity with investment was evaluatedqualitatively by visual inspection of disks and copings prior to andafter sand-blasting of the reaction layer. Surfaces of the disks wereinspected for pittings under low-magnification (8×) stereomicroscope.Compositions comprising combinations of Ta₂O₅, and CeO₂ were found tohave the best combination of high translucency (low opacity) and lowreactivity with the investment.

TABLE 3 Oxide, wt % Ex. 6* Ex. 16 Ex. 17 Ex. 18 Ex. 19 Ex. 20 Ex. 21 Ex.22 Ex. 8* Ex. 23 SiO₂ 68.8 68.12 68.5 68.2 67.5 67.8 67.9 68.0 67.2 67.5B₂O₃ 1.3 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 Al₂O₃ 4.8 4.7 4.8 4.7 4.74.7 4.7 4.7 4.7 4.7 CaO 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 BaO 2.82.7 2.8 2.7 2.7 2.7 2.7 2.7 2.7 2.7 Li₂O 14.4 14.3 14.3 14.3 14.1 14.214.2 14.2 14.1 14.1 K₂O 2.2 2.2 2.2 2.2 2.2 2.2 2.2 2.2 2.2 2.2 Na₂O 1.51.4 1.5 1.4 1.4 1.4 1.4 1.4 1.4 1.4 P₂O₅ 3.3 3.3 3.3 3.3 3.3 3.3 3.3 3.33.2 3.3 Y₂O₃ — 1.04 — — — — 1.04 — — — CeO₂ — — 0.39 — — — 0.39 0.390.39 0.39 Tb₄O₇ — — — 0.85 — — — 0.85 — — Ta₂O₅ — — — — 2.02 — — — 2.01— La2O3 — — — — — 1.5 — — 1.49 Motar ratio of 1.777 1.765 1.777 1.7651.765 1.765 1.765 1.765 1.765 1.765 (Na₂O + K₂O + CaO + SrO +BaO):(ZnO + Al₂O₃) 3-pt Flexure 420 ± 440 ± Strength per ISO 60 60 6872,MPa As-Pressed Rod 3- 370 ± 370 ± pt Flexure 40 30 Strength, MPaAs-pressed 42 35 39 37 27 25 31 32 25 24 Opacity (relative opacityunits) Reactivity with — Medium Lower Higher Medium Higher Lower MediumLower Highest investment material *Correspond to compositions in Table2.

Pellets of the compositions of Examples 2, 6, 8 and 9 were used to pressa variety of dental articles in the AutoPress® dental press(Jeneric/Pentron, Wallingford, Conn.) at pressing cycles carried outunder vacuum and involving heating from 700° C. to 920° C. and holdingthe temperature for 20 minutes prior to initiation of the pressingcycle. Pressure of 0.5 MPa was applied for 7 minutes through amold-plunger assembly schematically shown in FIG. 1. The plungerassembly used to press the pellets into dental restorations may be thesystem set forth in copending commonly assigned application Ser. No.09/431,659, filed Nov. 1, 1999, now U.S. Pat. No. 6,302,186, which ishereby incorporated by reference. Disks of compositions of examples 11and 13 were pressed as described above and chemical solubility wasmeasured according to ISO 6872 and found to be significantly lower thanthe acceptable limit of 100 μg/cm².

EXAMPLE 2

Three different powders of lithium disilicate glass-ceramics given inTable 4 below were used to make jacket crowns. One powder was formedfrom the composition of Example 24, a second powder was formed fromfifty percent of the composition of Example 25 and fifty percent of thecomposition of Example 26, and a third powder was formed from thecomposition of Example 26. The powders were mixed with water to a thickpaste consistency and were applied to refractory dies made form PolyvestRefractory Die Material available from Whip Mix Corp., Louisville, Ky.and Synvest Refractory Die Material available from Jeneric/Pentron Inc.,Wallingford, Conn. Both investments were found adequate. Cores werebuilt up in two applications and fired as given below. The firstapplication was fairly thin. Fired cores were sectioned, polished with120 and 400 grit sandpaper. Polished cross-sections were studied usingan optical microscope at magnifications of 50× and 200×. Cores werefound to be fully dense. Only occasionally pores smaller than 30 um wereobserved. Some of the copings were built up to full crowns using OPC®3G™ porcelain available from Jeneric/Pentron Inc., Wallingford, Conn.and found to be more than adequate in aesthetics and function.Additionally, the powder of composition 26 was wet-condensed into bars.The bars were fired and polished as per ISO-6872. Three-point bendtesting was conducted on ten bars and the flexure strength was measuredto be 241±25.

TABLE 4 26 (= 11 from composition 24 25 Table 3) oxide Wt % Mole % Wt %Mole % Wt % Mole % SiO₂ 68.7 64.08 70.6 64.73 68.8 63.68 B₂O₃ — — 0.90.71 1.2 1.00 Al₂O₃ 4.8 2.64 4.8 2.59 4.8 2.62 ZnO 0 0.00 0 0.0 0 0.00MgO 0 0.00 0 0.0 0 0.00 SrO 0 0.00 0 0.0 0 0.00 CaO 1.0 1.0 0.5 0.49 1.00.99 BaO 2.8 1.02 1.4 0.50 2.8 1.02 Li₂O 14.4 27.01 14.7 27.10 14.426.80 K₂O 2.2 1.31 2.1 1.23 2.2 1.30 Na₂O 1.5 1.36 1.4 1.24 1.4 1.30ZrO₂ 0 0.00 0 0.00 0 0.00 TiO₂ 0 0.00 0 0.00 0 0.00 P₂O₅ 3.3 1.30 3.61.40 3.3 1.29 Tb₄O₇ 0.7 0.05 0 0.00 0 0.00 CeO₂ 0.7 0.23 0 0.00 0 0.00Molar Ratio of 1.78 1.34 1.76 (Na₂O + K₂O + CaO + SrO + BaO)/(Al₂O₃ +ZnO) Firing 890° C. × 890° C. × 890° C. × Temperature 1 min hold 1 minhold 1 min hold

The glass-ceramics of the present invention have the capability to beused to fabricate dental articles using powder application techniques,pressing techniques or machining techniques to provide single ormulti-unit dental restorations at temperatures below about 950° C. usingalready existing, commercially available equipment such as the Autopressavailable from Jeneric/Pentron, Wallingford, Conn. Pressability orability to flow and be pressed into complex shapes of dentalrestorations at these temperatures is achieved due to the presence of asufficient amount of the residual glass in the resulting glass-ceramic,in the range of about 15%-60% by volume. The glass-ceramics of thepresent invention have the capability to be shaded by admixing pigmentsto the glass-ceramic powder by methods commonly used for dentalporcelains.

As will be appreciated, the present invention provides a simple andeffective method for producing lithium disilicate glass-ceramiccompositions and dental restorations therefrom. While variousdescriptions of the present invention are described above, it should beunderstood that the various features can be used singly or in anycombination thereof. Therefore, this invention is not to be limited toonly the specifically preferred embodiments depicted herein.

Further, it should be understood that variations and modificationswithin the spirit and scope of the invention may occur to those skilledin the art to which the invention pertains. Accordingly, all expedientmodifications readily attainable by one versed in the art from thedisclosure set forth herein that are within the scope and spirit of thepresent invention are to be included as further embodiments of thepresent invention. The scope of the present invention is accordinglydefined as set forth in the appended claims.

What is claimed is:
 1. A dental restoration comprising in weightpercent: about 62 to about 85% SiO₂; about 1.5 to about 10% Al₂O₃; about8 to about 19% Li₂O; about 2.5 to about 7% K₂O; about 0.5 to about 12%P₂O₅; up to about 7% CaO; up to about 5% ZnO; up to about 1% SrO; up toabout 5% Na₂O; up to about 7% BaO; and wherein the molar ratio of(Na₂O+K₂O+CaO+SrO+BaO)/(Al₂O₃+ZnO)≧1.3.
 2. The dental restoration ofclaim 1 manufactured from a glass-ceramic that is pressable.
 3. Thedental restoration of claim 1 further comprising in weight percent: upto about 4.9% B₂O₃; up to about 1.5% F; up to about 2% MgO; up to about5% Cs₂O; up to about 2% TiO₂; up to about 3% ZrO₂; up to about 1% SnO₂;up to about 1% Sb₂O₃; up to about 3% Y₂O₃; up to about 1% CeO₂; up toabout 1% Eu₂O₃; up to about 1% Tb₄O₇; up to about 2% Nb₂O₅; and up toabout 2% Ta₂O₅.
 4. The dental restoration of claim 1 formed into acomponent selected from orthodontic appliances, bridges, spacemaintainers, tooth replacement appliances, splints, crowns, partialcrowns, dentures, posts, teeth, jackets, inlays, onlays, facing,veneers, facets, implants, abutments, cylinders, and connector.
 5. Thedental restoration of claim 1 manufactured from a blank of glass ceramicmaterial.
 6. The dental restoration of claim 1 manufactured from aglass-ceramic formable powder.
 7. A glass-ceramic composition comprisingin weight percent: about 64 to about 70% SiO₂; about 1.5 to about 6Al₂O₃; about 10 to about 15% Li₂O; about 2.5 to about 5% K₂O; about 2 toabout 7% P₂O₅; up to about 7% BaO; up to about 1% SrO; up to about 2%ZnO; up to about 0.9% CaO; up to about 3% Na₂O; and wherein the molarratio of (Na₂O+K₂O+CaO+SrO+BaO)/(Al₂O₃+ZnO)≧1.3.
 8. The glass-ceramiccomposition of claim 7 whereby the glass-ceramic is pressable.
 9. Theglass-ceramic composition of claim 7 further comprising in weightpercent: up to about 1.5% F; up to about 5% Cs₂O; up to about 2.7% B₂O₃;up to about 2% MgO; up to about 2% TiO₂; up to about 3% ZrO₂; up toabout 1% SnO₂; up to about 1% Sb₂O₃; up to about 3% Y₂O₃; up to about 1%CeO₂; up to about 1% Eu₂O₃; up to about 1% Tb₄O₇; up to about 2% Nb₂O₅;and up to about 2% Ta₂O₅.
 10. A dental restoration comprising theglass-ceramic of claim
 7. 11. A glass-ceramic composition comprising inweight percent: about 62 to about 85% SiO₂; about 5.1 to about 10 Al₂O₃;about 8 to about 19% Li₂O; about 0.5 to about 12% P₂O₅; up to about 5%Na₂O; up to about 7% K₂O; up to about 7% CaO; up to about 1% SrO; up toabout 7% BaO; up to about 5% ZnO; and wherein the molar ratio of(Na2O+K₂O+CaO+SrO+BaO)/(Al₂O₃+ZnO)≧1.3.
 12. The glass-ceramiccomposition of claim 11 whereby the glass-ceramic is pressable.
 13. Theglass-ceramic composition of claim 11 further comprising in weightpercent: up to about 1.5% F; up to about 5% Cs₂O; up to about 4.9% B₂O₃;up to about 2% MgO; up to about 2% TiO₂; up to about 3% ZrO₂; up toabout 1% SnO₂; up to about 1% Sb₂O₃; up to about 3% Y₂O₃; up to about 1%CeO₂; up to about 1% Eu₂O₃; up to about 1% Tb₄O₇; up to about 2% Nb₂O₅;and up to about 2% Ta₂O₅.
 14. A dental restoration comprising theglass-ceramic of claim
 11. 15. A glass-ceramic composition comprising inweight percent: about 64 to about 70% SiO₂; about 5.2 to about 9 Al₂O₃;about 10 to about 15% Li₂O; about 2 to about 7% P₂O₅; up to about 3%Na₂O; up to about 5% K₂O; up to about 0.9% CaO; up to about 1% SrO; upto about 7% BaO; up to about 2% ZnO; and wherein the molar ratio of(Na₂O+K₂O+CaO+SrO+BaO)/(Al₂O₃+ZnO)≧1.3.
 16. The glass-ceramiccomposition of claim 15 whereby the glass-ceramic is pressable.
 17. Theglass-ceramic composition of claim 15 further comprising in weightpercent: up to about 1.5% F; up to about 5% Cs₂O; up to about 2.7% B₂O₃;up to about 2% MgO; up to about 2% TiO₂; up to about 3% ZrO₂; up toabout 1% SnO₂; up to about I% Sb₂O₃; up to about 3% Y₂O₃; up to about 1%CeO₂; up to about 1% Eu₂O₃; up to about 1% Tb₄O₇; up to about 2% Nb₂O₅;and up to about 2% Ta₂O₅.
 18. A dental restoration comprising theglass-ceramic of claim
 15. 19. A glass-ceramic composition comprising inweight percent: about 64 to about 70% SiO₂; about 1.5 to about 6 Al₂O₃;about 10 to about 15% Li₂O; about 2 to about 7% P₂O₅; about 2.2 to about5% K₂O; about 0.5 to about 3% Na₂O; about 0.5 to about 3% B₂O₃; up toabout 0.9% CaO; up to about 1% SrO; up to about 7% BaO; wherein themolar ratio of (Na₂O+K₂O+CaO+SrO+BaO)/(Al₂O₃)≧1.3.
 20. The glass-ceramiccomposition of claims 19 whereby the glass-ceramic is pressable.
 21. Theglass-ceramic composition of claims 19 further comprising in weightpercent: up to about 1.5% F; up to about 5% Cs₂O; up to about 2% TiO₂;up to about 3% ZrO₂; up to about 1% SnO₂; up to about 1% Sb₂O₃; up toabout 3% Y₂O₃; up to about 1% CeO₂; up to about 1% Eu₂O₃; up to about 1%Tb₄O₇; up to about 2% Nb₂O₅; and up to about 2% Ta₂O₅.
 22. A dentalrestoration comprising the glass-ceramic of claim
 19. 23. Aglass-ceramic composition comprising in weight percent: about 62 toabout 76% SiO₂; about 1.5 to about 10 Al₂O₃; about 8 to about 19% Li₂O;about 0.3 to about 7% P₂O₅; about 0.5 to about 8% Ta₂O₅; up to about 5%Na₂O; up to about 7% K₂O; up to about 7% CaO; up to about 1% SrO; up toabout 7% BaO; up to about 5% ZnO; wherein the molar ratio of(Na₂O+K₂O+CaO+SrO+BaO)/(ZnO+Al₂O₃)≧1.3.
 24. The glass-ceramiccomposition of claim 23 whereby the glass-ceramic is pressable.
 25. Theglass-ceramic composition of claim 23 further comprising in weightpercent: up to about 5% B₂O₃. up to about 1.5% F; up to about 2% MgO; upto about 5% Cs₂O; up to about 2% TiO₂; up to about 3% ZrO₂; up to about1% SnO₂; up to about 1% Sb₂O₃; up to about 3% Y₂O₃; up to about 1% CeO₂;up to about 1% Eu₂O₃; up to about 1% Tb₄O₇; and up to about 2% Nb₂O₅.26. A dental restoration comprising the glass-ceramic of claim
 23. 27. Amethod of making a lithium disilicate dental restoration comprising:melting a starting glass composition at temperatures within the range ofabout 1200 to about 1600° C.; quenching the glass melt; subjecting thequenched glass to one or more heat treatments in the temperature rangeof from about 400° to about 1100° C. to convert the glass into aglass-ceramic; comminuting the glass ceramic to a powder; forming thepowder onto a die to form a dental restoration; and sintering the formeddental restoration.
 28. The method of claim 27 wherein the glass-ceramiccomposition comprises: about 62 to about 85% SiO₂; about 1.5 to about10% Al₂O₃; about 8 to about 19% Li2O; about 0.5 to about 12% P₂O₅; up toabout 7% CaO; up to about 5% ZnO; up to about 1% SrO; up to about 5%Na₂O; up to about 7% BaO; and wherein the molar ratio of(Na₂O+K₂O+CaO+SrO+BaO)/(Al₂O₃+ZnO)≧1.3.
 29. The method of claim 27wherein the glass-ceramic composition comprises: about 64 to about 70%SiO₂; about 1.5 to about 6 Al₂O₃; about 10 to about 15% Li₂O; about 2.5to about 5% K₂O; about 2 to about 7% P₂O₅; up to about 7% BaO; up toabout 1% SrO; up to about 2% ZnO; up to about 0.9% CaO; up to about 3%Na₂O; and wherein the molar ratio of(Na₂O+K₂O+CaO+SrO+BaO)/(Al₂O₃+ZnO)≧1.3.
 30. The method of claim 27wherein the glass-ceramic composition comprises: about 62 to about 85%SiO₂; about 5.1 to about 10 Al₂O₃; about 8 to about 19% Li₂O; about 0.5to about 12 % P₂O₅; up to about 5% Na₂O; up to about 7% K₂O; up to about7% CaO; up to about 1% SrO; up to about 7% BaO; up to about 5% ZnO; andwherein the molar ratio of (Na₂O+K₂O+CaO+SrO+BaO)/(Al₂O₃+ZnO)≧1.3. 31.The method of claim 27 wherein the glass-ceramic composition comprises:about 64 to about 70% SiO₂; about 5.2 to about 9 Al₂O₃; about 10 toabout 15% Li₂O; about 2 to about 7% P₂O₅; up to about 3% Na₂O; up toabout 5% K₂0; up to about 0.9% CaO; up to about 1% SrO; up to about 7%BaO; up to about 2% ZnO; and wherein the molar ratio of(Na₂O+K₂O+CaO+SrO+BaO)/(Al₂O₃+ZnO)≧1.3.
 32. The method of claim 27wherein the glass-ceramic composition comprises: about 64 to about 70%SiO₂; about 1.5 to about 6 Al₂O₃; about 10 to about 15% Li₂O; about 2 toabout 7% P₂O₅; about 2.2 to about 5% K₂O; about 0.5 to about 3% Na₂O;about 0.5 to about 3% B₂O₃; up to about 0.9% CaO; up to about 1 % SrO;up to about 7% BaO; and wherein the molar ratio of(Na₂O+K₂O+CaO+SrO+BaO)/(Al₂O₃)≧1.3.
 33. The method of claim 27 whereinthe glass-ceramic composition comprises: about 62 to about 76% SiO₂;about 1.5 to about 10 Al₂O₃; about 8 to about 19% Li₂O; about 0.3 toabout 7% P₂O₅; about 0.5 to about 8% Ta₂O₅; up to about 5% Na₂O; up toabout 7% K₂O; up to about 7% CaO; up to about 1% SrO; up to about 7%BaO; up to about 5% ZnO; and wherein the molar ratio of(Na₂O+K₂O+CaO+SrO+BaO)/(ZnO+Al₂O₃)≧1.3.